1. Field of the Invention
The present invention relates to a magnetic detector for detecting changes of an applied magnetic field, and more particularly to a magnetic detector suitable for detecting, e.g., rotation information of an internal combustion engine.
2. Description of the Related Art
Generally, a giant magnetoresistance device (referred to as a GMR device hereinafter) is the so-called artificial lattice film, i.e., a laminate manufactured by alternately forming a magnetic layer and a non-magnetic layer one above the other in thickness of several angstroms to several tens angstroms, as described in "Magnetoresistance Effect of Artificial Lattice", Journal of Applied Magnetism Society of Japan, Vol. 15, No. 51991, pp. 813-821. Such known artificial lattice films are represented by (Fe/Cr)n, (Permalloy/Cu/Co/Cu)n, and (Co/Cu)n. The GMR device exhibits a much greater MR effect (MR change rate) than a conventional magnetoresistance device (referred to as an MR device hereinafter). Also, the GMR device is the so-called in-plane magnetic sensitive device of which MR effect depends on only a relative angle between the directions of magnetization of the magnetic layers adjacent each other, and which produces the same changes in resistance value regardless of any angular difference in direction of an external magnetic field with respect to a current.
In this respect, there is known a technique for detecting changes of a magnetic field as follows. Magnetic sensitive surfaces are formed by GMR devices, and electrodes are provided at both ends of each magnetic sensitive surface to form a bridge circuit. A constant-voltage and constant-current power supply is connected between two opposing electrodes of the bridge circuit so that changes in resistance value of the GMR devices are converted into voltage changes, thereby detecting changes of the magnetic field acting on the GMR devices.
FIG. 7 is a view showing a construction of a conventional magnetic detector using a typical GMR device as mentioned above; FIG. 7A is a side view and FIG. 7B is a plan view.
The conventional magnetic detector comprises a rotary member of magnetic material (referred to as a plate hereinafter) 2 which has projections capable of changing a magnetic field and is rotated in synch with a rotary shaft 1, a GMR device 3 arranged with a predetermined gap relative to the plate 2, and a magnet 4 for applying a magnetic field to the GMR device 3. The GMR device 3 has magnetoresistance patterns 3a, 3b formed in its magnetic sensitive surface. Furthermore, the GMR device 3 is attached in place by a fixing member (not shown) of non-magnetic material with a predetermined gap relative to the magnet 4.
In the above construction, when the plate 2 rotates, the magnetic field applied to the GMR device 3 is changed and so does a resistance value of each magnetoresistance pattern 3a, 3b.
FIG. 8 is a block diagram of a circuit configuration of a conventional magnetic detector.
The conventional magnetic detector comprises a Wheatstone bridge circuit 11 using GMR devices which are arranged with a predetermined gap relative to a plate 2 and are subject to a magnetic field applied from a magnet 4, a differential amplification circuit 12 for amplifying an output of the Wheatstone bridge circuit 11, a comparison circuit 13 for comparing an output of the differential amplification circuit 12 with a reference value, and a waveform shaping circuit 14 for receiving an output of the comparison circuit 13 and outputting a signal having a level "0" or "1" to an output terminal 15.
FIG. 9 shows one specific example of the circuit configuration represented by the block diagram of FIG. 8.
The Wheatstone bridge circuit 11 includes GMR devices 10A, 10B, 10C and 10D which are each disposed, by way of example, in one side of a bridge. One ends of the GMR devices 10A and 10C are interconnected at a junction point 16 which is connected to a power source terminal Vcc, while one ends of the GMR devices 10B and 10D are interconnected at a junction point 17 which is grounded. The other ends of the GMR devices 10A and 10B are interconnected at a junction point 18, while the other ends of the GMR devices 10C and 10D are interconnected at a junction point 19.
The junction point 18 of the Wheatstone bridge circuit 11 is connected to an inverted input terminal of an amplifier 12a in the differential amplification circuit 12 through a resistor. The junction point 19 is connected to a non-inverted input terminal of the amplifier 12a through a resistor and also connected through a resistor to a voltage dividing circuit which constitutes a reference power supply.
Further, an output terminal of the amplifier 12a is connected to an inverted input terminal of the comparison circuit 13. A non-inverted input terminal of the comparison circuit 13 is connected to a voltage dividing circuit which constitutes a reference power supply, and also connected to an output terminal thereof through a resistor.
An output terminal of the comparison circuit 13 is connected to the power source terminal Vcc through a resistor, and a base of a transistor 14a in the waveform shaping circuit 14. A collector of the transistor 14a is connected to the output terminal 15 and also connected to the power source terminal Vcc through a resistor, whereas an emitter of the transistor 14a is grounded.
The operation of the above magnetic detector will be described below with reference to FIG. 10.
When the plate 2 rotates, the GMR devices 10A and 10D of the Wheatstone bridge circuit 11 are subject to the same changes of a magnetic field, and the GMR devices 10B and 10C thereof are subject to the changes of a magnetic field which are the same to each other, but different from the changes of a magnetic field applied to the GMR devices 10A and 10D, corresponding to projections and recesses of the plate 2 shown in FIG. 10A. As a result, resistance values of the pairs of GMR devices 10A, 10D; 10B, 10C are changed corresponding to the projections and recesses of the plate 2 such that the resistance values are maximized and minimized in reversed positional relation. Middle point voltages at the junctions 18, 19 of the Wheatstone bridge circuit 11 are also changed likewise.
Then, a difference between the middle point voltages is amplified by the differential amplification circuit 12 and, as shown in FIG. 10B, an output V.sub.D0 indicated by a solid line is produced at the output terminal of the differential amplification circuit 12 corresponding to the projections and recesses of the plate 2 shown in FIG. 10A.
The output of the differential amplification circuit 12 is supplied to the comparison circuit 13 and compared with a comparison level, i.e., a reference value V.sub.TH. A comparison signal is shaped in waveform by the waveform shaping circuit 14. Consequently, an output having a level "0" or "1", indicated by a solid line in FIG. 10C, is obtained at the output terminal 15.
In the conventional magnetic detector, however, a large gain cannot be achieved because changes in resistance value of each GMR device are reduced due to the temperature coefficient of the resistance value of the GMR device. Accordingly, there has been a problem that the conventional magnetic detector is easily affect ed by noise and has lower noise resistance.